The present invention relates to a method for de-icing the engine, wing and empennage systems, or control surfaces, of aircraft utilizing electrical resistance heaters which are supplied with heating energy in succession, the quantity of the heating energy fed to the resistance heaters being controlled by establishing heating and nonheating periods.
It is known to energize the resistance heaters disposed at the surfaces to be de-iced in a continuous manner until the ice present at these surfaces has been completely melted. The drawback of this process is that excess heat supplied and/or stored in the heating elements as well as the flow of air lead to a flow of the water thus formed back over unheated surfaces, resulting in renewed, and uncontrollable, ice formation.
This drawback is avoided by another known electrothermal de-icing process in that the electrical resistance heaters are heated in succession only for short periods. This merely melts the adhesion layer between the ice and the aircraft structure so that the ice pieces which then float on the surface of the aircraft structure are removed by the centrifugal and/or aerodynamic forces occurring during flight. With this process it is thus possible to de-ice completely the engine, wing and empennage systems of aircraft.
A de-icing system operating according to the last-mentioned process is disclosed in U.S. Pat. No. 3,420,476, issued to Wolfgang Volkner et al on Jan. 7, 1969. In this system the heat required for de-icing is generated in electrical resistance heaters which are arranged in groups, some of the resistance heaters being constantly provided with heating energy and other resistance heaters of the same group being supplied with heating energy for short intervals in succession and in a defined sequence. A first clock pulse generator with a given constant clock pulse frequency generates energizing pulses for those resistance heaters which are to be temporarily supplied with heating energy while a second clock pulse generator furnishes de-energizing signals, or pulses, between every two successive energizing pulses at a time which depends on the temperature of one of the resistance heaters. In order to control the de-energizing signals, the temperature at the surface of one of the constantly energized resistance heaters is used.
In this known de-icing system, temperature sensors disposed in the constantly heated strips where the ice breaks off or at the surface of the resistance heaters embedded in these strips make it possible to monitor the environmental conditions such as temperature, air pressure, humidity and relative velocity of the air streams at the surfaces to be de-iced and their effect on the temperature of the heating mats and the heating strips.
FIGS. 1a and 1b of the accompanying drawing illustrate a heating sequence in the form of a timing diagram and a temperature vs. time curve, respectively, for a known de-icing system including a total of ten resistance heaters 1 to 10. Referring to FIG. 1a, the heating time, t.sub.H, under environmental conditions which are not explained in detail, is assumed to be 100 seconds for all ten resistance heaters, each heater 1-10 thus having a heating time, T.sub.Hn, where n is the reference numeral identifying the particular element, of 10 seconds, while a nonheating time, t.sub.p, of 140 seconds is provided. The cycle time, t.sub.z, is thus 240 seconds. When the environmental conditions change, the cycle time can be lengthened or shortened to correspond to such changes by lengthening or shortening the nonheating time t.sub.p. This is effected in the known de-icing system by a manual adjustment made by the pilot. However, it is extremely difficult for the pilot to accurately adapt the nonheating period to the environmental conditions since he must perform a number of additional tasks, particularly at a time when the environmental conditions are changing.
FIG. 1b shows the temperature behavior at the resistance heater 1, which also applies for the subsequent resistance heaters 2 to 10. Resistor 1 is heated as well as cooled according to an exponential function, FIG. 1b showing the temperature curve for the heating time t.sub.H1 as a solid line and the temperature curve for the cooling period as a descending dashed line. Experiments have shown that in the extreme case a heating period t.sub.H1 of 10 seconds creates a rise in temperature of approximately 53.degree. C. on the surface of resistance heater 1 while the cooling period t.sub.A1 required to return to the starting temperature then is about 100 seconds so that the resistor will have reached its starting temperature again after a period, P, of about 110 seconds from the start of heating. The point of intersection of the temperature curve for the cooling period of the resistor with the time axis is at P.
It is generally known that the amount of water present in the atmosphere, with the smallest water particles being able to be cooled to about -43.degree. C. and suddenly turning to ice if there is a change in their surface tension to thus produce critical icing on aircraft, principally depends on the temperature, type of clouds, size of the field of clouds and size of the water droplets in the clouds. Thus, for example, at a temperature of 0.degree. C. a stratus cloud may contain 0.06 g water per m.sup.3 and a cumulus cloud 3.84 g water per m.sup.3. It has also been found by way of flight experiments that temperatures between +5.degree. C. and +10.degree. C. are required on the surface of a helicopter rotor blade to melt the adhesive layer between the ice and the surface, according to the above-described process, so that whole ice pieces can be removed as a result of centrifugal and/or aerodynamic forces.
The momentarily existing meteorological conditions thus determine the cooled liquid water content and thus, with sufficiently constant flow speed at the engine, wing and empennage systems of the aircraft, the ice accumulation rate.
In the example depicted by FIGS. 1a and 1b, tripling of the ice accumulation rate would have the result that the mass of ice permissible on the aircraft surfaces would be exceeded since the heating period can be extended only by 240/100 = 2.4 &lt; 3. On the other hand, a rising ambient temperature, and thus a rise in the starting temperature, would result in a shift of the point of intersection P to after the beginning of a new heating cycle. In the known de-icing system this would result in a progressive increase in the temperature at a heating resistance until the ice would melt continuously and renewed uncontrollable ice formation could develop on the unheated surfaces.